[ItBites]

What you are suggesting won't make much difference since the cross section of the thin sheet metal brackets is small. The thin, small cross-section has a hard time flowing heat from the mounting bolts to the tubular portion of the log. The materials being suggested cannot be made sturdy enough in the same thin cross-section, so you will have to increase their cross-section to mount the tubular log. This will increase the heat flux through those mounts and even though they have a lower heat conductivity, the extra cross-section will increase the total conductivity of the mount bracket. I could run some numbers, but I suspect the fuel log is hot from convective heating from the engine compartment air and maybe some radiation from the manifold and valve covers and less driven by the heat flux through the mounting brackets. As a test, you could put some phenolic washers on either side of the sheet metal brackets (and lightly bolt them back in place), which should significantly reduce the conductive heat flux into the bracket. If this solves your problem, you can make a permanent fix.

For a real fix, though, I recommend you put in a return line to the tank, with a regulator. Then, when you get back in the car and begin to pump fuel, the hot fuel in the log will flow back to the tank and be replaced with cooler fuel in an instant. Old cars with old designs had vapor lock and other problems with fuel systems and it sounds like you emulated the old system. That's fine, but you will also be subjected to little annoyances like this.

PS, the fuel volume in the log doesn't help. The fuel flows very slowly in there and is likely nearing full temperature before shutting the car off, so you're probably on the edge of this condition all the time. Decrease fuel stagnation and allow a return so the fuel is not deadheaded and you won't have any issues.